There have been used DC arc furnace as shown in FIG. 1 to melt scrap material.
As shown in FIG. 1, a DC arc furnace comprises a furnace shell 2 with a lower electrode 1 (anode) at its bottom, a furnace roof 3 adapted to close an upper portion of the furnace shell 2, an upper electrode 4 extending through the roof 3 and movable vertically, a lower conductor 5 connected to the lower electrode 1 and extending radially, an upper conductor 6 connected to the upper electrode 4 and extending radially and a power source circuit 10 including a transformer 8 and a rectifier 9 both arranged between the extension ends of the upper and lower conductors 5 and 6 and connected to an AC power source 7.
In an operation of melting scrap or the like material 11, the roof 3 is moved upward and removed outside to open the upper portion of the furnace shell 2. Using a bucket or the like, the material 11 is charged into the furnace shell 2. Then, the top of the furnace shell 2 is closed with the roof 3 and heavy-current is excited across the electrodes 1 and 4 by the power source circuit 10. While arc 12 is generated between the electrodes 1 and 4, the upper electrode 4 is gradually lowered to melt the material 11.
However, the conventional DC arc furnace as shown in FIG. 1 has a problem of the arc 12 between the electrodes 1 and 4 being generated in a deflected direction.
This will be explained more specifically. In the upper and lower conductors 6 and 5, which are respectively connected to the upper and lower electrodes 4 and 1 and extend radially, the upper conductor 6 is arranged to be vertically movable further above the roof 3; whereas the lower conductor 5 must be arranged at a position near the arc 12 to be generated since no great space is allowed below the furnace shell 2. Therefore, a strong magnetic field 13 generated by the heavy-current flowing through the lower conductor 5 acts on the arc 12. The magnetic field 13 is generated right-handedly in accordance with right-hand screw rule by the heavy-current flowing through the lower conductor 5 to the lower electrode 1 as shown by the arrows.
Since the electric current flowing upward in the upper electrode 4 as shown by the arrow due to the arc 12 crosses the magnetic field 13, the arc 12 receives force F in a direction toward a furnace peripheral wall 14, i.e., in a direction (to the left) away from the radial direction along which the the lower conductor 5 extends, in accordance with the Fleming's left-hand rule on the basis of the (upward) direction of the electric current and the direction (perpendicular to the sheet of FIG. 1 and from front to back of the sheet) of the magnetic field 13. As a result, the material 11 charged to the center of the furnace shell 2 is melted positively merely in the deflected direction of the arc 12 and is hardly melted at a side away from the deflected direction so that unmelted material is left and/or a hot spot or spots are generated, resulting in nonuniform temperatures in the furnace shell and extreme lowering of the melting efficiency.
The arc 12, which is deflected, tends to be directed to the furnace peripheral wall 14, resulting in damages of the wall. In order to prevent the peripheral wall 14 from being damaged, enough distance must be retained between the upper electrode 4 and the furnace peripheral wall 14, which disadvantageously causes a problem of the furnace shell 2 being increased in size.
The present invention was made to overcome the above problems encountered in the prior art and has for its object to provide a DC arc furnace in which two upper electrodes are provided to be connected to their respective power source circuits and arcs generated are directed to the center of the furnace shell, thereby enhancing the melting efficiency of the material, preventing the furnace peripheral wall from being damaged and worn and compacting the furnace shell in size. Because of the deflected degree of the arcs being changeable, concentrated melting of unmelted material can be facilitated and hot spots are eliminated.